The struggle of
Daphnia water
fleas and their
parasites have
been studied.

Adding parasites to the mix

Next they
introduced several species of parasite, 200 of each, whose
power depended on "virulence genes" matched by "resistance
genes" in the hosts. The least resistant hosts and the
least virulent parasites were killed in each generation.
Now the asexual population no longer had an automatic
advantage -- sex often won the game. It won most
often if there were lots of genes that determined resistance
and virulence in each creature.

In the model,
as resistance genes that worked would become more common,
then so too would the virulence genes. Then those resistance
genes would grow rare again, followed by the virulence
genes. As Hamilton put it, "antiparasite adaptations are
in constant obsolescence." But in contrast to asexual
species, the sexual species retain unfavored genes for
future use. "The essence of sex in our theory," wrote
Hamilton, "is that it stores genes that are currently
bad but have promise for reuse. It continually tries them
in combination, waiting for the time when the focus of
disadvantage has moved elsewhere."